What Are Shorelines?

• Shorelines are the interfaces between land and water bodies, such as oceans, seas, and lakes.
• They are dynamic environments, continuously shaped by physical processes like wave action, erosion, deposition, and sea-level change
• Shorelines evolve due to natural forces and human interference.
• Understanding coastal processes is essential for safety, infrastructure, and minimising the consequences of human modifications.

Wave Formation

• Waves are created by wind energy transferred to the surface of water.
• Three key factors determine wave size:
  • Wind speed
  • Duration of wind blowing
  • Fetch: the distance over water that the wind blows
• The longer and stronger the wind, and the larger the fetch, the bigger the waves.

Key Wave Parameters

• Crest: The highest point of a wave
• Trough: The lowest point of a wave
• Wavelength (λ): Horizontal distance between two successive crests (or troughs)
• Amplitude: Vertical distance from the midpoint to a crest (or trough)
• Wave height: Total vertical distance from crest to trough (2 × amplitude)
• Wave velocity: Speed at which wave crests move across the water

Wave Motion

• In deep water, water particles move in nearly circular orbits.
• These circular motions do not move the water forward—only the energy travels.
• The size of the orbital motion decreases with depth.
• At a depth equal to half the wavelength, water movement stops — this is the wave base.

What is the Wave Base?

• The wave base is the depth in water below which there is no wave motion
• It is equal to ½ the wavelength
• In deep water, waves do not interact with the seafloor
• In shallow water (depth < wave base), waves “feel the bottom” and begin to slow and steepen

What Happens as Waves Approach Shore?

• As water gets shallower:
  • Waves slow down
  • Wavelength decreases
  • Wave height increases
  • Waves become steeper and break
• This results in surf zones, where waves release energy on the coast and break.
• This process is essential for erosion and sediment transport along shorelines.

Wave Breaking & Longshore Currents

• Waves usually approach the shore at an angle, not straight on.
• As the wave reaches shallow water, the part of the wave that touches bottom slows down first.
• The rest of the wave, still in deeper water, keeps moving faster and bends (refracts), becoming nearly parallel to the shore.
• As the wave slows, its wavelength decreases and wave height increases—eventually causing it to break.

Longshore Current Formation:

• Even after refraction, waves still hit the shore at a slight angle.
• Each breaking wave pushes water along the beach, generating a longshore current.
• This current flows parallel to the shoreline within the surf zone, transporting sediments and reshaping coasts.

Longshore Current: Erosion & Deposition

• The movement of the longshore Current leads to longshore drift which is the zigzag transport of sediments by swash and backwash.
• Longshore drift is a combined effect of the longshore current and swash/backwash movement that transports sediment along the coast.
  • Longshore drift moves a tremendous amount of sediment along coasts, and it is responsible for creating a variety of depositional features.
• Swash: The upward movement of water onto the beach.
• Backwash: The downward return of water back to the ocean.
• Together, they move sediment in a zigzag pattern along the beach.

Rip Currents

• A rip current is a strong, narrow flow of water moving away from the shore.
• It forms in the nearshore area and returns water pushed to shore by incoming waves.
• Flows perpendicular to the shoreline, usually through breaks in sandbars
• Fed by longshore currents that accumulate water in the surf zone
• Dissipates just beyond the surf zone
• May be fed by longshore currents from both directions when no dominant flow exists
• Rip currents are dangerous to swimmers: they can quickly carry people away from the shore.
  
• Swimmers caught in one should swim parallel to the shore to escape.

Rip Currents: example

• Note: If part of a beach does not have a strong unidirectional longshore current, the rip currents may be fed by longshore currents going in both directions.

Landforms Created by Coastal Erosion

Headlands and Bays

• Headlands: Projections of harder rock that resist erosion.
• Bays: Areas of softer rock that erode more easily, forming curved inlets.
• See this video on YouTube

Sea Cliffs

• Steep coastal faces formed by persistent wave attack at the base of slopes or rock walls.

Wave-Cut Platform

• A flat bench of rock at the base of sea cliffs.
• Formed as cliffs retreat due to wave erosion in the surf zone.

Sea Caves → Arches → Stacks

• Sea Caves: Created when waves erode along zones of weakness (e.g., joints or bedding planes).
• Arches: Formed when two caves meet or break through a headland.
• Stacks: Isolated pillars left behind after an arch collapses.

Coastal Straightening

• Waves refract and focus energy on headlands, eroding them faster than bays.
• Over time, coastlines become straighter and smoother.

Evolution of a straightened coast

Wave-Cut Platform

Stacks

Arch and Stack

Landforms Created by Coastal Deposition

• Beaches
• Spit
• Baymouth Bar
• Tombolo
• Barrier Island
• Carbonate Reefs

Beaches

• Accumulation of sand or gravel along the shoreline.
• Beach face: Zone between low and high tide.
• Berm: Flatter zone above high tide, flooded only during storms.

Spit

• A narrow ridge of sand extending into open water.
• Formed by longshore drift transporting sediment down the coast.

Watch this video on YouTube: Spit formation by longshore drift

Baymouth Bar and Tombolo

Baymouth Bar

• A spit that crosses the mouth of a bay, partially or completely enclosing it.
• May still allow outflow of freshwater streams.

Tombolo

• A sand bar that connects an island to the mainland.
• Forms in the wave shadow where energy is lower behind an island.

Barrier Island

• A long, narrow island of sand parallel to the mainland.
• Acts as a protective barrier from ocean waves and storms.
• Common along tectonically stable coasts (e.g., U.S. Gulf Coast).

Watch this video on YouTube: What is a Barrier Island?

Carbonate Reefs

• Found in tropical regions (30° N to 30° S), in shallow, clear marine waters.
• Thrive where there is low input of clastic sediment and warm temperatures (~25°C).
• Built by corals, algae, and shelled organisms.
• Enhanced by upwelling currents that supply nutrients.
• Reefs absorb wave energy and protect back-reef zones where carbonate fragments accumulate.